12.Graphene Nanoelectronics and Silicon CMOS Technology the end of the Roadmap

G. Fiori, G. Iannaccone

Collaborations: University of Tarragona (Spain), University of Catania, University of Bologna, University of Calabria, University of Illinois at Urbana-Champaign (US), University of Granada (Spain)

13.Nanopower electronics

S. Di Pascoli, G. Iannaccone

Collaborations: University of Calabria

SILICON NANOWIRES: INNOVATIVE DEVICES AND LARGE AREA ARRAYS FOR ENERGY HARVESTING

G.Pennelli, M.Piotto, A.Nannini, M.Macucci

Devices based on silicon nanowires (SiNWs) have been fabricated by means of a top down process, based on high resolution electron beam lithography, silicon anisotropic etching and stress controlled oxidation. The process, developed on Silicon on Insulator substrates, is fully compatible with standard C-MOS technology and allows the fabrication of Nanowires as small as 10 nm and several micrometers long. The flexibility in fabrication offered by the employed process is exploited for the simultaneous fabrication of nanowires together with contacts and control gates for the investigation of electrical transport. The possibility of integrating complex devices on a single silicon nanowire has been investigated. In particular, a technique based on electron beam stimulated oxide etching has been improved and an accurate calibration has been performed by means of AFM investigation. This technique has been used as a precision “scalpel” for removing the oxide by well localized points on silicon nanowires. By exploiting this technque, localized windows through the oxide surrounding SiNWs have been opened and used for selective doping of silicon nanowires. In this way, pn nanojunction have been fabricated along the nanowire; electrical characterization of these pn nanojunction has been performed and reported[1].

Mechanical properties of SiNW, and related electrical transport, have been also investigated [2].

One of the future application field for nanodevices will be direct conversion of thermal to electrical energy. Thermoelectric devices, based on nanostructures as silicon nanowires, show an increased figure of merit, with respect to bulk materials, for the decrease of thermal conductivity due to the low dimensionality. A technology for the fabrication of SiNW arrays (meshes) has been investigated. At the moment, it is possible to fabricate SiNWs, as small as 30 nm, organized on large area arrays (more than 1 mm2). The SiNW density is higher than 106 nanowires per mm2 [2]. Future work will be dedicated to the measurement of electrical transport and thermal properties of SiNW large area arrays, and solutions for their application to energy harvesting will be investigated.

The applications of innovative electronic systems in the fields of new generation vehicles and nautical environment are key research themes of the Electronic Systems Lab. The results of the research activities, published in [1-12], show how electronics can be fundamental for (i) active safety thanks to driver assistance devices such as rear-view fish-eye camera [2] or micromirror-based head-up display [5], (ii) new LED-based automotive lighting systems [6,7], (iii) advanced gearbox mechatronic actuators [4] or in-vehicle distributed control [3,8], (iv) the successful realization of low-emission vehicles [1], either electric, hybrid or gas/hydrogen fuelled, by solving one of the main issues in this field: the safe and efficient control of the energy accumulators utilized to store the on-board propulsion power [9-11]. As proved in Electronics has a key role to improve the quality of the industrial production processes of luxury yachts [12] and provides innovative sensors for the measurements of the seawater parameters [13].

The research on mixed-signal and RF/mm-wave circuits carried out at the RF lab and at the Electronic Systems lab of the University of Pisa has been focused on:

- Design using 65nm CMOS Silicon on Insulator (SOI) technology of innovative on-chip antennas and LNA for 60 GHz and 77 GHz wireless systems (wireless Gbit short-range communications or automotive radar). At such high frequency the wavelength is short (few mm) and the antenna can be integrated on-chip: a double slot antenna has been designed with a 4 dB gain, co-optimization of the integrated antenna and LNA (no more 50 Ohm impedance constraint) allows for the design of an LNA with 20 dB gain and limited power consumption [1-4]

The research on Assistive Technologies carried out at the Electronic Systems lab of the University of Pisa has been focused on:

- A mobility aid system for visually impaired people for autonomous mobility in urban outdoor scenarios [1]. Based on users' requirements identified in collaboration with the UIC - Lucca, and thanks to the collaboration with ISTI-CNR Pisa, a set of electronic devices has been integrated onto a traditional white cane (smart cane); the smart cane is able to signal the user that he/she is moving on a safe predefined path thanks to a vibration feedback provided to the user when the path is detected by the smart cane tip, which occurs in the range of a few dozens centimeters from the path centre. The safe path is implemented by means of an electrical circuit buried or laying on the ground and a proper signal driver. Starting from GPS data provided by a wireless GPS receiver and from the ON/OFF path detection status, a smartphone unit provides the user with guidance function by means of prerecorded audio messages containing information about the current location, the path structure and the surrounding environment. The system has been tested on the Walls of Lucca city by target users.

- A microcontroller based device has been realized to allow people with upper limbs disabilities and mild cognitive disabilities to play with a very popular videogame console [2]. The device allows users with upper limbs impairments to control one or more functions of the game, by means of commercially available sensors for the exploitation of specific user's residual abilities; the device may be used in connection with the original joypad, so depending on the game itself and the user's capabilities, both autonomous game and collaborative games scenarios are possible.

- ElGo, is an electro-mechanical soccer goalkeeper for a five-a-side football goal. It is composed by a dummy moved on a slide, placed near the goal line, thanks to a belt and a DC motor, and remotely controlled by a disabled player by means of controls specific to exploit his/her residual capabilities, e.g. switch controls, joysticks, etc. [3]

- e(asy)Phone is a mobile phone user interface specifically designed to easy access to Android mobile phones by people with motor impairments [4].

The research on DSP and communication hardware-software platforms carried out at the Electronic Systems lab of the University of Pisa has been focused on:

- Innovative Network-on-Chip (NoC) and many cores System-on-Chip architectures for green computing in applications ranging from mobile multimedia to cloud servers. To this aim new IP macrocells [1,2], MPSoC architectures [3,4], verification methodologies [5] have been developed and patented in close collaboration with STMicroelectronics.

- A new platform H@H for remote monitoring of patients affected by chronic heart failure has been defined within the EU project Health@Home [6]. Wireless bio sensors [7] are connected to a smart gateway [8] in charge of data collection, DSP for signal quality improvement and early clinical warning and communication through a specific protocol [9] with the Hospital Information System.

- DSP and communication platforms (based on IC [10] or FPGA [11,12]) have been also developed in the framework of ESA and INFN projects to increase the networking and computation capabilities of electronic systems operating in harsh radiation environments such as space and nuclear physics experiments.

The research activity carried out by the micro-nano system group of the Pisa Unit was focused on four main subject summarized below.

Solid state directional anemometers. An improved version of the original directional anemometer has been developed. The wind range has been extended using an active section based on a 5-microchannel pressure sampling configuration. A real single chip - two dimensional device, equipped with a wireless interface for data transmission have been fabricated and fully characterized in a wind-tunnel.

Thermal flow sensors: A novel offset cancellation technique have been demonstrated by means of a discrete component interface applied to double-heater thermal flow meter. The flow sensors were fabricated by post-processing of chips designed with a BCD process of STMicroelectronics. The sensor offset compensation is obtained by applying a proper power unbalance between the two heaters of the microstructure. Differently from conventional offset cancellation approaches, the proposed method produces also a strong reduction of the offset temperature drift.

Acoustical Particle Velocity Sensors (APVS): These sensors measures the local velocity field developed by acoustic waves. The advantage of these detectors with respect of traditional microphones is the possibility, when combined with the latter, to obtain a full knowledge of the acoustic field. This aspect is important to implement advanced functions such as sound source localization, active noise suppression and sound recording in near field conditions. The devices are based on a thermal detection mechanism, already exploited in the so called MicroFlown probes. Differently from the latter, the proposed sensors are fully CMOS compatible and are based on a much more robust structure. These characteristics are crucial to expand the application field of APVS and dramatically reduce fabrication costs.

Silicon carbide Neutron Spectrometers. The alfa particle detectors, on which the neutron spectrometer is based, have been improved in order to make them suitable for integration with the neutron converter layer (Lithium Fluoride). Preliminary test with a macroscopic neutron converter have been successfully performed.

ANALOG INTEGRATED INTERFACES

P.Bruschi, M. Piotto, F.Butti

The activity in this field has been focused on the development of a low power interface circuit for thermal integrated sensors. The interface is composed of a low noise-low offset instrumentation amplifier and a power efficient, programmable differential heater driver.

As far as the instrumentation amplifier is concerned, the novel architecture introduced in previous year has been further improved in order to make it feasible for integration into a compact CMOS cell. The architecture is a chopper amplifier merged with a Gm-C filter, performing both offset cancellation and bandwidth limiting functions. The circuit incorporates original solutions, such input port swapping to reduce gain error, improving the overall precision. Input common mode control has been introduced to limit serious drawbacks of the port-swapping approach, making it applicable also in the case of large input common mode swing.

The differential heater driver has been designed to implement an original offset compensation technique, so far demonstrated only with discrete component boards. The driver allows fine tuning of the output current differential mode and variation of the total delivered power in both (continuous) analog and digital way. Particular care have been devoted to obtain very low differential and common mode current noise, in order not to degrade the resolution of the sensors.

The interface, including the instrumentation amplifier, the heater driver and a programmable digital conhrol unit has been integrated into a smart sensor designed with the BCD6s process of STMicroelectronics.

APPLICATION OF THE DIRAC EQUATION TO THE STUDY OF TRANSPORT IN GRAPHENE

M. Macucci

We have used the Dirac equation to investigate transport in graphene nanoribbons and flakes, obtaining new results in terms of PT-symmetry breaking, and preparing a detailed review paper.

In particular, we have developed [1] a numerical solver of the Dirac equation for a single-layer graphene nanoribbon with armchair edges and with a longitudinally constant external potential (which can then be used as a building block for a general transport simulator for graphene devices with arbitrary potentials), pointing out that, because of the spinorial nature of the wave function, some properties of graphene nanoribbons can be described by means of an effective non-Hermitian PT-symmetric Hamiltonian, although there is no dissipation.

We have provided numerical evidence for the PT-symmetry breaking and given an order parameter. Finally, we have studied the behavior of eigenmodes and eigenfunctions in the neighborhood of exceptional points.

We have also prepared a review [2], in the first part of which we provide a general description of the k • p method, a semi-empirical approach which allows to extrapolate the band structure of materials from the knowledge of a restricted set of parameters evaluated in correspondence of a single point of the reciprocal space. The k • p method is treated both in the case of homogeneous crystals (where we have considered a formulation based on the standard perturbation theory, and Kane's approach) and in the case of nonperiodic systems (where, following Luttinger and Kohn, we have described the single-band and multi-band envelope function method and its application to heterostructures). The second part of the review is completely devoted to the application of the k • p method to graphene and graphene-related materials. Following Ando's approach, we have shown how the application of this method to graphene results in a description of its properties in terms of the Dirac equation. Then we have found general expressions for the probability density and the probability current density in graphene and we have compared such a formulation with alternative existing representations. Finally, applying proper boundary conditions, we have extended this treatment to carbon nanotubes and graphene nanoribbons, recovering their fundamental electronic properties.

The research activity for 2011 focused mainly on two topics: shot noise suppression due to carrier recombination in p-n junctions, and shot noise and diffusive transport in nanoelectronic devices.

Concerning shot noise in p-n junctions, we have performed [1] a theoretical and experimental investigation of its suppression in gallium arsenide and silicon diodes as a result of generation-recombination phenomena. In particular, the availability of the cross-correlation technique and of ultra-low-noise amplifiers has allowed us to significantly extend, down to 10 pA, the range of bias current values for which results were available in the literature. We have observed full shot noise at low bias currents, at which the generation-recombination centers are almost empty (and therefore no significant correlation is introduced among carriers), suppressed shot noise at intermediate current levels, at which traps are half filled, and full shot noise again at high current levels, where traps are almost completely filled and therefore their contribution to the total current becomes negligible. To provide a quantitative understanding of this behavior, we have extended the Shockley-Read-Hall model for the trap-assisted generation-recombination mechanism. Such a model has represented the theoretical background for the performed Monte Carlo noise simulations, which have provided good agreement with the experimental results.

From the point of view of noise in nanoelectronic devices, we have numerically investigated [2] the conductance and the Fano factor in mesoscopic conductors, obtained by modulation doping in GaAs/AlGaAs heterostructures containing randomly located scatterers. In our simulations we represent these scatterers, deriving from the presence of randomly located impurities and dopants, either with hard-wall obstacles or with realistic potential fluctuations. Our results show that in nanoelectronic semiconductor devices it is quite unlikely to reach the diffusive regime, mainly due to the insufficient number of propagating modes.

We have also presented, as an invited paper at ICNF 2011, an update [3] on shot noise suppression in a series of cascaded barriers, showing that the well-known diffusive limit reported in the literature on the basis of semiclassical models can be achieved only in the presence of a mechanism leading to mode mixing, such as a magnetic field. Without mode mixing, strong localization appears, because the localization length is of the order of the mean free path. These results are consistent with existing experimental data on shot noise in superlattices.

Gas Sensors Based on Modification of Integrated Solid State Devices by Mesostructured Porous Silicon

G. Barillaro, L. M. Strambini

The past decade has been characterized by a high degree of technological advance in the field of gas sensors. Old [e.g., polymers, metal oxides, porous silicon (PS)] and new (e.g., carbon nanotubes) materials have been used for the development of standard (e.g., resistive, capacitive) and innovative [mostly field effect transistor (FET)-like] sensors, able to detect a number of volatile and hazardous compounds (e.g., CO, CO2, NO2). Recently, gas sensor fabrication by modification of integrated solid-state devices trough the use of mesostructured PS (mPS) has been proven to be very effective in terms of compatibility with integrated circuit (IC) commercial processes as well as of modeling/optimization of gas sensor devices.

In Ref. [1], a new approach for driving integrated FET gas sensors, performing a current-voltage conversion, and, simultaneously, reducing power dissipation is presented. The proposed approach is experimentally tested on an integrated Porous Silicon JFET (PSJFET) sensor upon exposure to hundreds ppb of NO2. The PSJFET is a p-channel JFET integrating a mesostructured PS layer between drain and source. The PS layer acts as a sensing gate and allows the JFET current to change upon adsorption/desorption of analytes. An electric gate (G) is also available and allows the JFET current to be electrically tuned independently of analytes. A negative feedback loop is used to modulate the gate (G) voltage in order to compensate for PSJFET source-drain current variation induced by adsorption of analytes in the sensing gate, and hence to maintain the sensor power dissipation at a constant, fixed value. The negative feedback loop also allows implementing an intrinsic current-voltage conversion of the sensor signal. Experimental results on the dynamic characterization of PSJFET driven according to the proposed power-saving approach are reported for several NO2 concentrations (100ppb, 300ppb, 500ppb), at room temperature operation.

Over the last decade, electrochemical processes have become increasingly important in microelectronic manufacturing. As an example, electrochemical deposition of copper (damascene process) for multilevel interconnections has become accepted for mainstream silicon chip manufacturing, especially for high-speed logic devices. There are many other opportunities for utilizing electrochemical processes in the manufacture of semiconductor devices and microsystems. For instance, anodization, that is the electrochemical oxidation of a material, can be either used to produce an oxide of the same material (electrochemical oxidation), as in the case of metals, or to etch the material itself (electrochemical etching), as in the case of silicon.

The ECM technology is actually based on the controlled anodic dissolution of silicon in aqueous electrolytes containing a few percent of hydrofluoric acid (HF), once a pattern is defined onto the silicon surface. Among the most interesting features of such a technology there are: i) the high (sub-micrometric) accuracy in deep etching the pattern defined onto the silicon surface; ii) the high uniformity (in plane and out-of-plane) and quality (roughness of tens on nanometers) of etched surfaces; iii) the high aspect ratio (over 100) of feasible structures; iv) the possibility of changing the etching anisotropy (from zero to one) during the etching itself; v) the possibility of simultaneously etching small and large areas (from a few μm^2 up to several mm^2) in the same chip; and finally vi) the low-cost of both equipment and chemicals, and the high fabrication flexibility.

The ECM technology has been successfully applied to the fabrication of a number of microsystems with different applications, so far, among which fabrication of high-order one-dimensional silicon/air photonic crystals working at 1.55 μm to be used as scaffold/transducer for growth/monitoring biological matters (e.g. amyloid fibrils) [1] as well as sensing element in optofluidic microsystems for chemical/biochemical analysis [2, 3].

In Ref. [1], authors investigate the use of a silicon micromachined structure, fabricated by electrochemical etching, as a three- dimensional supporting matrix also suitable for optically monitoring the amyloid fibrils growth. The biology and the structure of amyloid fibrils are under extensive investigation in many laboratories: they are co- causative agents of diseases such as Parkinson’s and Alzheimer’s. The silicon device consists in a periodic array of silicon walls with high aspect-ratio. This periodic arrangement of silicon and air gives rise to one- dimensional hybrid photonic crystals, suitable for out-of-plane (top view) imaging but, potentially, also for in-plane label-free testing. Preliminary results relative to fluorescence microscopy analysis are performed to investigate the interaction among silicon microstructures and fibrillar proteins. Samples of the highly amyloidogenic variant of human β2- microglobulin (P32G β2-m) are deposited on flat silicon dice as well as inserted into the gaps of the micromachined silicon devices. After Thioflavin T labeling, a bright emission originating only from silicon devices where polymerized amyloid fibrils are present is observed.

In Ref. [2, 3], fabrication and testing of an optofluidic microsystem exploiting high aspect-ratio, vertical, silicon/air one-dimensional (1D) photonic crystals (PhC) are reported. The microsystem is composed of an electrochemically micromachined silicon substrate integrating a 1D PhC featuring high-order bandgaps in the near-infrared region, bonded to a glass cover provided with inlet/outlet holes for liquid injection/extraction in/out the PhC-itself. Wavelength shifts of the reflectivity spectrum of the pho- tonic crystal, in the range 1.0–1.7 micron, induced by flow of different liquids through the PhC air gaps are successfully measured using an in-plane all-fibre setup, thanks to the PhC high aspect-ratio value. Experimental results well agree with theoretical predictions and highlight the good linearity and high sensitivity of such an optofluidic mi- crosystem in measuring refractive index changes. The sensitivity value is estimated to be 1,049 nm/RIU around 1.55 micron, which is among the highest reported in the literature for integrated refractive index sensors, and explained in terms of enhanced interaction between light and liquid within the PhC.

Meso and nanostructured materials are extensively studied in view of their peculiar electrical, optical, and mechanical properties for applications in many fields, from electronics to optoelectronics, from photovoltaics to biomedicine and environment monitoring. Among such materials, mesostructured and nanostructured silicon obtained by low-cost anodization of either p or n-type crystalline silicon, namely porous silicon (PS), has been successfully employed for the fabrication of integrated gas sensors, exploiting the high sensitivity of PS electrical and optical properties to gas and vapor adsorption, and, more recently, for the fabrication of high-efficiency solar cells, exploiting the fine tuning of PS refraction index for producing antireflection coatings and increasing light absorption.

In Ref. [1], fabrication, electrical characterization, and modeling of fully porous pn junctions (FPJs) consisting of elemental mesoscopic crystalline junctions operating in parallel, is presented. FPJs are fabricated by anodic etching of a pn crystalline substrate and show a rectifying behavior strongly dependent on PS surface termination, as proved by electrical measurements performed after both room-temperature aging and thermal-oxidation treatment. Modeling of FPJs is performed using a lumped equivalent circuit consisting of a diode, taking into account the elemental mesoscopic junctions, and two resistances acting one in series and one in parallel to the diode, the latter taking into account conduction paths at the silicon mesocrystal surface. Best fitting of experimental data results in good agreement between theoretical and measured I-V curves, thus corroborating the proposed model. Future work is in progress for controlling the etching process so as to produce a regular array of high-density mesoscopic pn junctions operating in parallel for sensing and photovoltaic applications.

In Ref. [2], remote detection of reactive analytes using optical films constructed from electrochemically prepared porous Si-based photonic crystals is demonstrated. Porous Si samples are prepared to contain either surface oxide or surface Si-H species, and analyte detection is based on irreversible reactions with HF(aq) or Cl2(g) analytes, respectively. HF dissolves silicon oxide from the porous matrix, causing an irreversible blue-shift in the resonance peak of the photonic crystal. Cl2 reacts with the native Si-H species present on the surface of as-etched porous Si to generate reactive silicon halides that evaporate from the surface and/or react with air to convert to silicon oxide. Either Cl2-related process reduces the net refractive index of the material that is detected as a blue shift in the spectrum. With sufficient analyte concentrations or exposure times, the spectral blue shifts are visible to the unaided eye. A portion of the porous nanostructure is filled with inert polystyrene, which acts as an internal spectral reference. The polymer fiducial protects that portion of the sensor from attack by the corrosive analytes. Reflectance spectra from both the polymer-filled and the unfilled, reactive porous layers are acquired simultaneously. The fiducial marker also allows elimination of arti- facts associated with shifts of the resonance peak upon changing the angle of incidence of the optical probe. Theoretical angle-resolved spectra (transfer matrix method) show a good match with the experimental data. High-temperature air or room-temperature ozone oxidation reactions are used to prepare the HF-reactive surface, and it is found that the ozone oxidation reaction produces a greater sensitivity to HF (LLOD of 0.1% HF in water).

Graphene Nanoelectronics and Silicon CMOS Technology the end of the Roadmap

G. Fiori, G. Iannaccone

Among the many unique structural and electronic properties of graphene, some are very promising for nanoelectronic applications, such as room temperature mobility much larger than that of bulk silicon. However, even assuming that many manufacturing challenges will be overcome with technology development, graphene also presents a serious problem and a possible showstopper for electronic applications: it has a zero bandgap.

Transistors and diodes need to be made of a semiconductor with a gap sufficiently large to suppress interband tunneling, that can undermine the possibility of switching the device off. For this reason, several options for inducing a gap in graphene have been pursued in recent years, with partial success, at least from the point of view of manufacturability of large scale integrated circuits.

Especially when a fabrication technology is in its infancy, as in the case of graphene, modeling can be a very powerful tool to evaluate the perspectives of different technology options. First, one has to be optimistic, and assume that fabrication techniques will improve to a point in which the ideal devices we dream of can be realized.

Counterfeiting severely affects different industrial sectors, including the pharmaceutical, the aircraft, the automotive, and the luxury goods Industry. For example, coun- terfeiting of pharmaceutical defrauds consumers and poses ill patients at severe health risks. Recently, radio frequency identification (RFID) technology has been considered in the United States by the Food and Drug Administration (FDA) with the aim of combating counterfeit pharmaceuticals. For this kind of applications it is crucial to be able to authenticate the object as the one originally issued by a given institution, bank or company. From this point of view, low-cost unforgeable authentication hardware would greatly broaden the range of applications of automatic authentication and identification.

We propose a silicon PUF (the “nanokey”) to be used in a challenge-response authentication scheme. The circuit exploits the variability of minimum size MOSFET threshold voltages in a standard CMOS 90 nm process, in order to generate a unique unclonable and reliable digital response to challenges provided as input to the system. This solution is also robust to invasive attacks because removing passivation layers would dramatically alter small differences between transistor threshold voltages.

We propose and use a series of circuits and algorithmic solu- tions that strongly suppress the dependence of circuit behavior and of the authentication operation upon process, temperature, supply voltage variations, and ageing. In this way we are able to obtain a lower BER and more robust operation, compared to the other solutions proposed in the literature. This first prototype has been used to verify the concept and the effects of aging on its performance. A second prototype has been implemented, to verify the operations of an authentication circuit with a much larger CRP space, and therefore much more resistant to brute force attacks.